Isotherm Study for the Biosorption of Cd (II) from Aqueous Solution by the Aquatic Weed: Ceratophyllum demersum

Owing to various anthropogenic activities our environment is being polluted especially by discharging effluents containing heavy metals. Heavy metals are nondegradable and persist for a long time in the environment. Hence they pose potential risk on the environment. Cadmium is a toxic element and cause severe damages to both animals and plants. Biosorption which is a process that utilizes dead or living biomass to sequester heavy metals is currently gaining considerable important as an alternative low cost and eco-friendly technology to remove heavy metals from aquatic media. In this study, three aquatic plants such as Eichhornia crassipes, Ceratophyllum demersum and Salvinia mollesta were screened for the biosorption of cadmium and the plant biomass was analyzed for Cd using Atomic Absorption Spectrometer. Ceratophyllum demersum was selected and batch experiments were carried out to examine the influence of biosorbent dosage, initial concentration of metal ion and contact time on the cadmium (II) ion adsorption. Sorption data was analyzed using linear and nonlinear regression methods of Langmuir and Freundlich isotherm models. Desorption experiment was also carried out. Biosorption of cadmium by Ceratophyllum demersum was very rapid (20 minutes) at pH 5 and at room temperature. Adsorption capacity of Ceratophyllum demersum for cadmium increased with increasing concentration of Cd ions. The nonlinear adsorption isotherm of Langmuir model did not fit well with the sorption data (high χ2 = 4.58). Although three linear forms (I, II and V) of Langmuir isotherm model supported the sorption data (R2=0.9316, 0.9124 and 0.912), however the separation factor, RL, was less than zero and also maximum adsorption capacity, qm were very low in all five linear forms. Both linear and nonlinear forms of Freundlich adsorption isotherm model exhibited a better fit with high correlation R2=0.92 and low χ2 = 1.95 respectively, which indicates a multilayer adsorption. Desorption experiments indicated that 0.1M EDTA, 0.1M HCl and 0.1M HNO3 were efficient desorbents for recovery of Cd (II) from the loaded biomass.


Introduction
The discharge of heavy metals into aquatic systems has become a serious problem of concern all over the world. Heavy metals are one of the most serious inorganic pollutants due to its toxicity, persistence and bioaccumulation in aquatic organisms (Esfahani et al., 2013). These pollutants are introduced into aquatic systems significantly as a result of various anthropogenic activities. Cadmium is a toxic element, non-essential for plants and animals and is usually associated with zinc in most rocks and soils (Sutherland and Venkobachar, 2013). It is widely used in the manufacture of paints, plastics, batteries and metal plating. It is known to accumulate in living organisms, especially in human kidneys and causing damage to the filtering mechanism. Thus the removal and recovery of cadmium is important to protect the environment.
Traditional treatment methods for removing heavy metals such as chemical precipitation, chemical oxidation and reduction, electrochemical methods, ion exchange and reverse osmosis are often ineffective, expensive and generate secondary wastages (Ahalya et al., 2003). More over these methods are specific to each metal and are not eco-friendly.
Biosorption has been now recognized as an effective alternate method for removing heavy metals from aqueous solutions. It can be defined as the property of certain biomass to bind and concentrate selected ions or molecules form aqueous solution (Volesky and Naja, 2007). The major advantages of biosorption over other traditional treatment methods are low cost, high efficiency of metal removal from dilute solutions, regeneration of biosorbent, possibility of metal recovery and ecofriendly.

Preparation of Biosorbent
Mature and healthy samples of E.crassipes, Ceratophyllum demersum and Salvinia mollesta were collected from Gampaha and Piliyandala area. Aquatic plants were washed thoroughly with tap water to remove soil particles and other impurities and then with distilled water. Plant samples were dried in air for two days then in an oven at 60 o C for 24 hours (Radojevic and Baskin, 1999). They were ground into powder. The biomass was passed through a sieve of pore size 500 m and stored in plastic containers.

Screening Experiment
In this study three aquatic plants were screened for biosorption capacity. Biomass of each plant (3.0 g) was thoroughly mixed with 100 cm 3 of 5.0 mg/L cadmium solution at pH 5.0 in a conical flask. The suspension was shaken on a mechanical shaker for 3 hours at 80 rpm. Biomass was separated from the metal solution by filtering through a filter paper. The biomass washed thoroughly and subjected to Cd analysis.

Dry Ashing Method
Biomass of each plant was air dried then, oven dried at 105 o C for 4 hours. A known weight (2.5 g) of each dried biomass was taken in a small crucible and placed in muffle furnace at 450 o C for 4 hours. After cooling, a drop of distilled water was added and then 5.0 cm 3 of conc. HNO3 was added to the ash. The ashed sample was slowly heated on a sand bath for 30 minutes at 120 -130 o C. To this 5.0 cm 3 of 30 % H2O2 was added with care in small amounts to avoid possible foaming depending on sample composition. The heating was continued at that temperature until a clear solution was obtained (Hoenig et al., 1998). After cooling of sample, the solution was made up to 100.0 cm 3 and analyzed for Cd using Atomic Absorption Spectrophotometer (Varian -AA 280FS).

Investigation of the Effect of Dosage of Biosorbent
Different dosages (0.05 g, 0.1 g, 0.2 g, 0.3 g, 0.4 g, 0.5 g, 1.0 g, 1.5 g, and 2.0 g) of biosorbent of the selected plant from screening experiment were thoroughly mixed with 100 cm 3 of 5.0 mg /L cadmium solution at pH 5 and at 29 o C in conical flasks. The suspensions were shaken on a mechanical shaker for 180 minutes at 80 rpm. This experiment was performed in triplicates. Biomass was separated from the metal solution by filtering through a filter paper and filtrate was subjected to residual Cd determination by Atomic Absorption Spectrophotometer (AAS). Biomass in distilled water served as control.

Investigation of the Effect of Contact Time on Cadmium Adsorption
The biomass (0.5 g) of the selected plant was taken into a conical flask with 100 cm 3 of 5.0 mg /L cadmium solution at pH 5.0 and 29 o C. The suspensions were shaken on a mechanical shaker at 80 rpm. The experiment was performed in triplicates and the suspensions were removed from the shaker at pre-determined time intervals (5 min, 10, 15, 20, 30, 45, 60, 90, and 120 minutes) and filtered. The filtrate was subjected to residual Cd determination by AAS. Biomass in distilled water served as control.

Investigation of the Effect of Initial Concentration of Cadmium Solution
The biomass of (0.5 g) of the selected plant from the screening experiment was added to 100.0 cm 3 of solution of different initial concentrations (5,10,20,40,60,80,100, and 120 mg/L) at pH 5.0 individually and mixed thoroughly at 29 o C in conical flasks. The suspension was shaken on a mechanical shaker for 3 hours at 80 rpm. The experiment was conducted in triplicates. Biomass was filtered and filtrate was analyzed for Cd content by AAS.
The Sorption data from this experiment were subjected to both Linear and nonlinear forms (using Solver) of Langmuir and Freundlich adsorption isotherm models.

Adsorption Isotherm Models
Adsorption isotherms are mathematical models which explain the behavior of adsorption species between liquid and solid phases. Langmuir isotherm model and Freundlich isotherm model are two important isotherm models which are commonly used by researchers.

Langmuir Isotherm Model
Langmuir model assumes that all adsorbing sites are identical and the probability of a molecule to adsorb is proportional to the concentration of the adsorbate in the fluid. It also assumes that adsorbing sites are not affected by the presence of other adsorbate molecules. The curve is linear for low concentration and at high concentration the isotherm bends to form a horizontal asymptote which corresponds to the condition where all of the adsorbing sites are filled with the adsorbate.
The mathematical expression of Langmuir isotherm model is: (1) Langmuir equation can be linearized in to five different forms as shown in Table 1.

Table 1. Linear Langmuir equations Isotherm
Linearized Equation Where Ce is equilibrium concentration of metal (mg/L), qe is the amount of the metal adsorbed (mg/g) by per unit mass of the biosorbent (g), qm and KL are Langmuir constants evaluated from slope and the intercept respectively of the linear plots of Langmuir.
Based on the further analysis of Langmuir equation, the dimensionless parameter of the equilibrium known as separation factor (RL) is expressed by: (2) Where Co is the initial concentration of adsorbents. The RL parameter is considered as a more reliable indicator in the adsorption. There are four probabilities for the RLvalues.

Freundlich Isotherm Model
This model is commonly used to describe the adsorption characteristics for the heterogeneous surface. That is a multilayer adsorption can be explained by using Freundlich isotherm model.
The mathematical expression of Freundlich isotherm model is: Where Kf is an indicator of adsorption capacity and 'n' is adsorption intensity. The linearized Freundlich equation is expressed as follows: 'n' in the Freundlich isotherm model indicates about the favorable condition of the adsorption. The values of 'n' in the range from 2-10 indicate good adsorption, 1-2 moderate adsorption and less than 1 indicates poor adsorption (Mirjana et al., 2012).

Desorption Experiment
For the desorption experiment, 0.5 g of selected aquatic plant was loaded with 50.0 cm 3 of 40 mg/L cadmium solution at pH 5.0. This experiment was carried out at room temperature 29 o C for 20 minutes of contact time. The residual solution was subjected for Cd analysis. Cadmium loaded biomass were collected, gently washed and dried in air. Then the biomass was shaken with various desorbing agents such as 0.1 M EDTA (Ethylenediaminetetraacetic acid), 0.1 M HCl and 0.1 M HNO3 solutions for 20 minutes at 80 rpm in the room temperature (29 o C). Concentration of Cd 2+ in the aqueous solution was determined by AAS.

Screening of Biosorbents
Ceratophyllum demersum adsorbed maximum cadmium from the aqueous solution, and it was selected as the best biosorbent ( Figure 1).

Effect of Biosorbent Dosage on Cadmium Adsorption
The percentage of adsorbed cadmium by Ceratophyllum demersum increases with increasing dosage of biosorbent and reached maximum (99%) at 1.0 g of dosage.
Percentage of adsorption remains constant at dosage higher than 1.0 g (Figure 2).
When the dosage of biosorbent was increased, the surface area also increased. As a result the active sites will increase; this will lead to increase in adsorption. At dosage higher than 1.0 g, there were not enough biosorbate molecules for available binding sites. A similar result has been reported for biosorption of cadmium on to H. verticillata where the saturation level reached at dosage higher than 0.1 g (Huang et al., 2009).

Effect of Contact Time
The influence of contact time on biosorption of Cd (II) ions on to Ceratophyllum demersum at 29 o C is shown in Figure 3. The contact time is one of the important parameters affecting the biosorption process. It is very useful to design a successful biosorption system. A very rapid metal uptake occurred within 10 minutes of contact time and the sorption process reached equilibrium by a period of 20 minutes in which the adsorption of Cd was 95 % at pH 5.
A similar result had been reported by Yoonaiwong et al. (2011), where Utricularia aurea which showed a rapid uptake of Cd within 5 minutes and equilibrium was reached within 80 minutes.

Effect of Initial Concentration
The extent of removal of heavy metals from aqueous solution depends on the initial concentration. Adsorption capacity of Ceratophyllum demersum for Cd adsorption at various initial concentrations of Cd is given in Figure 4.

Figure 4: Effect of initial concentration of Cd on removal efficiency by
Ceratophyllum demersum (Contact time 180 minutes; temperature 29 °C; biomass dosage 2.0 g; pH 5; shaking speed 80 rpm) Adsorption capacity increases with increasing initial concentration continuously from 8 mg /L to 90 mg/L.
The sorption data was employed to non-Linear and Linear Langmuir isotherm models and the results are given below.

Linear Forms of Langmuir and Freundlich Isotherm Models
The different forms of Langmuir linear isotherm models were plotted in Figure 5-9. Table 2 shows the calculated isotherm parameters of Langmuir isotherm model.  The linear isotherm model For Freundlich isotherm was plotted in Figure 10. Table 3 shows the parameters of Freundlich Isotherm. However the separation factor RL was very low (RL < 0) for all five types of Langmuir isotherm models. Biosorption process will be favorable where RL is in between 0 -1. It is also observed that the qm value for all linearized form of Langmuir model were very low.

Figure 10: Freundlich Isotherm Model
Linearized form of Freundlich isotherm model showed a high R 2 value (0.9158) and adsorption intensity 'n' was 2.44 which indicate a good adsorption.
When compared to all five forms of linearized Langmuir isotherms with linearized Freundlich isotherm, Langmuir Type I, II and V have higher R 2 values. But Langmuir forms III and IV have same but very low R 2 values compared to Freundlich isotherm model ( Table 2).
Because of these different outcomes it is difficult to estimate the isotherm parameters by using only the linearized equations. Therefore nonlinear forms of both isotherm models were compared.

Nonlinear form of Langmuir and Freundlich isotherm models
Nonlinear form of Langmuir and nonlinear form Freundlich isotherm model is shown in Figure 11 and Figure 12 respectively. χ 2 values of both models are given in Table 4.   When compared χ 2 values of both models, the low value (1.95) for χ 2 for Freundlich isotherm suggests that this model can be used to explain the biosorption of Cd (II) from aqueous solution by the aquatic weed: Ceratophyllum demersum. (Mirjana et al., 2012)

Desorption Studies
To evaluate the efficiency and feasibility of a metal removal process, the regeneration and reversibility of the biosorbent should be considered. In this desorption study, 0.1M EDTA, 0.1M HCl and 0.1M HNO3 were used as the desorbing agent and the results ( Figure 13) indicate that EDTA, HCl and HNO3 desorbed 97 %, 94 %, and 85 % cadmium ions respectively from the biomass. Therefore, all EDTA, HNO3 and HCl could be used as desorbents for cadmium ions.

Conclusion
Non-living biomass of Ceratophyllum demersum is a good biosorbent for removing Cd (II) from aqueous solution.
Batch adsorption experiments showed that the adsorption of cadmium from aqueous solution is very rapid. The maximum percentage removal was 95% by 1.0 g of biomass at pH 5 such adsorption reached the equilibrium by 20 minutes. The adsorption of cadmium increases with increasing dosage of biosorbent. Cadmium ions adsorbed on Ceratophyllum demersum were effectively desorbed by 0.1M EDTA (97%), 0.1M HCl (94%) and 0.1M HNO3 (85%), which shows that adsorbed cadmium can be recovered from the loaded biomass.
Equilibrium studies showed that Freundlich isotherm model relatively showing better fit with the sorption data, which suggests a multilayer adsorption.